Years ago, few people would have considered carbon dioxide a pollutant. After all, it is necessary for plant life and is part of the cycle that keeps life alive on Earth. But given the scientific consensus that too much carbon dioxide will have a negative effect on the planet's climate and on public health and welfare, it makes sense to consider the gas as a pollutant that should be regulated by the federal Environmental Protection Agency.

Which is exactly what the Supreme Court did this week in its first major ruling on climate change - in the process sending a stern rebuke to the Bush administration. The court ruled - albeit narrowly, 5-4 - that the EPA has the authority to regulate carbon dioxide emissions from vehicles and power plants under the provisions of the Clean Air Act.

The EPA had been resisting such enforcement, and the Bush administration had argued that carbon dioxide emissions aren't pollution as defined by the act. But, as the ruling noted, the agency did not give a sound reason for that lack of aggressiveness.

"EPA has offered no reasoned explanation for its refusal to decide whether greenhouse gases cause or contribute to climate change," Justice John Paul Stevens wrote for the majority. He also wrote that although the decision does not require the EPA to determine that carbon dioxide poses a danger, the agency must base its decision on the law and on science.

Although the issue may seem to address a fairly narrow issue, the ruling has wider implications and could add to the urgency of climate change legislation from Congress, which House Democrats have promised to approve by July 4. That's good because the response to climate change must come at the federal level; allowing each state to develop its own standards would lead to a hodgepodge that companies would find difficult to meet and probably wouldn't be very effective.

Momentum continues to build on climate change, especially following a United Nations report summary released earlier this year, the full details of which should be released over the next several months. Climate change is happening; human activities are contributing to the change; the federal government needs to respond. The high court has taken a step forward. Now,it's time for the executive and legislative branches to follow. To read the Supreme Court's majority and dissenting opinions in Massachusetts v. EPA, go to www.supremecourtus.gov and click on "recent decisions."